Laboratory simulations have been carried out to test the possibility that interactions of SO2 and silicates can produce Na-S compounds which can account for the observed surface enrichment of Na relative to Si on Io. A wide variety of silicate compositions were heated under oxidizing conditions with a SO2/O2=1 gas phase at a mid-level crustal temperature for Io (1123 K). All experiments produced sulfate deposits on the silicate surfaces. The nature of the sulfate changed systematically with the silicate Ca/Na ratio, with mixtures of CaSO4 and Na-rich sulfate formed by basaltic compositions having higher Ca/Na but only alkali-rich sulfates formed for more granitic (low Ca/Na) compositions. For crystalline albite and an albite-orthoclase eutectic glass composition, K and Al-rich sulfates were formed. Assuming that burial of SO2 to the temperatures of our experiments is plausible on Io and that somewhat less oxidizing conditions do not qualitatively change the results, SO2-rock interactions producing Na-rich sulfates could account for the Io surface enrichment in Na. Observation on minor elements, such as K or Ca, in the atomic cloud or in magnetospheric ion spectra could be used to identify sulfates as Io surface phases and, conceivably, even define Ionian crustal magma types. Many analyses show significant S excesses which are best interpreted as due to the presence of bisulfate (HSO4) components, and NaHSO4 deserves consideration as an Io surface mineral. The possibility of elemental S on Io can be regarded as a totally separate issue from the problem of the surface Na enrichment. ¿ American Geophysical Union 1993